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The broad-line radio galaxy story Ilse van Bemmel European Southern Observatory, Karl-Schwarzschild-Str.2, D–85748 Garching Kapteyn Astronomical Institute, P.O.Box 800, NL–9700AV Groningen 1 0 0 2 Abstract n a J In this paper I discuss the issue of the so-called 25µm–peakers, which were dis- 1 covered with IRAS, and consist almost solely of broad-line radio galaxies. I find 1 that this peak is caused by the absence of colder dust that emits at 60µm and not 1 by an excess of hot dust, as suggested by the name. On optical images, the 25µm v peakers show a lack of extended dust. The peakers are consistent with being in a 7 7 later evolutionary stage, in which all the extended dust has been formed into stars 1 and the dust torus is smaller. If the 60µm dust is heated by stars, there might be 1 0 a correlation between the star-formation rate and the power of the AGN. 1 0 / Key words: galaxies: active, galaxies: infrared, quasars: infrared, galaxies: ISM h p - o r t 1 A prelude s a v: The first galaxies with nuclear activity were studied in detail by Seyfert in the i 1940’s (Seyfert 1943). Now, over 50 years later, their numbers have increased X by several orders of magnitude, revealing a rich diversity. In their attempt r a to reach a physical understanding of active galaxies, people have proposed unifiedschemes (e.g.Barthel1989),whichcombineseveralapparentlydifferent classes into one, depending on the orientation of a central symmetry axis. A dusty torus causes anisotropy at wavelengths were the dust is optically thick, but is transparent to hard X-rays and radio emission. This easily explains many differences between Seyfert1 and Seyfert2 galaxies, and also between radio galaxies and radio quasars. However, there are many open questions, e.g. the presence and nature of type2 radio-quiet quasars, the absence of broad emission lines in some objects (e.g. Hill et al. 1996). In this paper I will study one specific problem, that of the 25µm peakers. These are broad- line radio galaxies (BLRGs) which have a clear peak in their spectral energy distribution around 25µm, whereas all other radio-loud AGN have this peak at longer wavelengths (60–100µm). Email address: [email protected] (Ilse van Bemmel). Preprint submitted to Elsevier Preprint 1 February 2008 Fig. 1. The SED in restframe for the BLRG 3C390.3 (peaker), and the SEDs of BLRG 3C332 (filled circles) and and NLRG 3C321 (plusses), both non-peakers. The 25µm points are marked in the plot. 2 The problem starts The properties of the obscuring torus have been modeled by Pier & Krolik (1992) and Granato & Danese (1994). Both groups conclude that the dust shouldbeopticallythininthefar-infrared,atwavelengths largerthan∼80µm. Combining this result with the unified models, one can predict that narrow- line radio galaxies (NLRGs) and radio quasars (QSRs) of comparable radio power should emit comparable far-infrared power. No conclusive result was reached when comparing NLRGs and QSRs in the far-infrared, either with IRAS (Heckman et al. 1994), or with ISO (van Bemmel et al. 2000). However, one would expect the general shape of the far-infrared SEDs to be the same for all objects that belong to one class, which is true for NLRGs and QSRs. Broadlineradiogalaxieswerediscoveredandclassifiedfirstinthe1960’s,when the first radio surveys were done. They are typically galaxies with a bright nucleus and extended radio structures (like NLRGs), but a quasar spectrum in the optical. In the light of unified models, there seems to be an easy way to fit them in; BLRGs can simply be intermediate angle quasars. The dust torus is seen at an angle that just obscures the central source (and therefore enables us to see the host galaxy), but part of the broad-line region is un-obscured. As discussed above, one would expect that the infrared SEDs are the same for NLRGs, BLRGs and QSRs. However, there is a group of BLRGs which have a peak in their SED at 25µm, instead of 60–100µm as in NLRGs and QSRs (see Fig. 1). This seems to be in contradiction with expectations of the unification schemes, unless these 25µm peakers are somehow different. So far no NLRGs or QSRs are know with a clear peak in their SED at 25µm. 2 Fig. 2. Optical HST images of 3C390.3 and 3C321. The black lines indicate the radio axis and the image scale (de Koff et al. 1996). 3 Searching for clues Data were obtained for a limited sample of BLRGs with ISOPHOT on board ISO (Lemke et al. 1996, Kessler et al. 1996), in the bands 12, 25, 60, 90 and 160µm, using the P1, P2, C1 and C2 detectors. The observations were made in raster mode, to avoid the known problems with chopping, which are especially serious for these faint sources. The reduction of the C1 and C2 data is described in van Bemmel et al. (2000), and the reduction of the P1 and P2 data will be described in a future paper (van Bemmel et al. 2001). From additional ISOPHOT and literature data, two samples were selected, one of 25µm peakers and one of normal NLRGs and BLRGs. Due to the limited amount of infrared data, the comparison sample consists largely of NLRGs. Data in other spectral regimes were extracted from the NASA/IPAC Extra- galactic Database (NED) and the literature. The exact references for all data will also be given in the future paper (van Bemmel et al. 2001). The two samples are listed in Table 1, which gives their classification, infrared powers at 25 and 60µm and radio power at 178MHz. 4 Lining up the facts FromhereIwillrefertotheBLRGswithapeakat25µmasthepeakersample, and the comparison sample as the non-peaker sample. For both samples literature data were studied from X-ray, optical, infrared and radio observations, with different observational techniques (imaging, photometry, spectroscopy and/or polarimetry). There are always objects lacking data, but in general there were no immediate differences to be found when comparing the average luminosities over this spectral range and the spectral shape. However, a closer 3 Name ID P25 P60 P178 Name ID P25 P60 P178 3C33.1 B 24.41 24.36 – 3C61.1 N 23.97 24.59 27.39 3C98 B 23.19 <23.15 25.92 3C79 N 24.95 25.40 27.52 3C109 B 25.78 25.36 27.60 3C111 B 24.01 24.16 26.20 3C234 B 25.29 25.14 27.38 3C321 N 24.84 25.30 26.31 3C382 B 23.78 23.82 – 3C327 N 24.80 25.16 26.96 3C390.3 B 24.36 24.15 26.50 3C332 B 23.99 24.36 26.63 3C445 B 24.32 24.21 26.16 3C381 B? 24.31 24.48 26.92 3C403 N 24.17 24.47 26.27 3C405 N 24.70 25.25 28.75 Average 24.45 <24.31 26.71 Average 24.42 24.80 26.99 Table 1 −1 LuminositydensitiesinWHz forthepeakersample(left)andthenon-peakersam- ple (right). Averages for each sample are given for comparison. The second column gives the nature of the object, B for BLRG and N for NLRG. comparison reveals that there is no evidence at all for a peak at 25µm, as suggested by the name, it rather seems that these objects display a lack of 60µm emission. This means there is less cold dust in 25µm peakers than in non-peakers. The 60µm dust can either be heated directly by the AGN (Hes et al. 1995) or by stars; many star-burst galaxies have a peak in their SED at 60µm (Calzetti et al. 2000). This leaves two options: either the peakers have weaker AGN, so the dust is not hot enough to radiate at 60µm, or there is not enough dust in the host galaxy to detect it. The first option can be tested by comparing the 178MHz power of the two samples. Since the 178MHz emission is isotropic, thus independent onthe amount of dust in the host galaxy andthe orientation of the object, the 178MHz power should be significantly weaker for peakers. This is not confirmed in this sample. The second option can be tested by comparing optical images. Data from the HSTsnapshotsurveyofthe3CRsample(deKoffetal.1996)wereusedtocom- pare the optical appearance of the two samples (see also Fig. 2). A blind test was done with three people to classify the galaxies as non-interacting/dust- free, interacting/dust-rich or unclear. It turned out that there is a good relation between the distortion of the host (including dust lanes) and the shape of the infrared SED. The peakers are predominantly normal, dust-free elliptical galaxies, and often reside in sparse environments. The non-peakers, however, display dust lanes (as seen in Centaurus A) and other typical signs of interac- tion. The dust is usually extended and often perpendicular to the radio axis, which excludes the AGN as being the dominant heating source. 4 5 How to solve the problem 5.1 Unification With two groups of BLRGs it might seem that the unification schemes no longer apply. However, there is an historical pitfall. In the past, an active galaxy was classified as a radio galaxy when the optical galaxy was visible. When it was only a point-like source, it would be called a quasi-stellar object. The names stuck, but with present day instruments, there are many QSRs with visible host galaxies. Also, nearby quasars would have been classified as BLRG. A radio study reveals that there are indeed two groups of BLRGs, one consisting of low-luminosity quasar counterparts and another of mis-oriented quasars. Only the last group are BLRGs which fit into unified models (Dennett-Thorpe et al. 2000). In the sample with 25µm peakers, there might be more of the first group, while the comparison sample consists mostly of objects from the second group. This does not contradict unified models, but it shows that there is more to it than just orientation. 5.2 Nuclear and extended dust In the HST images the peakers lack extended dust related to star-formation, but there is no clear evidence that a central torus is absent as well. The fact that the broad-line region is visible indicates that the torus could be smaller in these objects. In the classical unification schemes there might be confusion between obscuration of the central parts by a nuclear torus, or by extended dust in the host galaxy. There is no easy way to tell the difference between these two geometries, but it implies that NLRG without extended dust could appear as BLRG, irrespective of the presence of a nuclear dust torus. 5.3 Evolution There might be an evolutionary link between 25µm peakers and normal BLRGs. Dust is the main fuel for both star-formation and the AGN, but there is a limited supply. Especially when the galaxy is isolated, there are no ways to re-fuel the host galaxy and star-formation will cease. When the fuel for the AGN runs out as well, one expects to see weaker emission lines and lower radio power. In addition, without extended dust, these galaxies should have little reddening and low polarization. Studies of the polarization, colors, stellar populations and X-ray absorption columns will provide more clues on the nature of the peakers. Hes et al. (1995) show that there is a correlation between 178MHz power and 60µm power, which would imply that the AGN heats the cold dust. However, this work indicates that in many objects the 60µm dust is related to star- formation processes, as in star-burst galaxies. Combining both results, there could be a relation between the star-formation rate in the host galaxy and the 5 power of the AGN. If one can confirm this relation in larger samples, it will provide important insights in the formation and evolution of both the host galaxy and the central black hole. 6 Live long and happy .... From observations with IRAS and ISOPHOT I find that there is a number of BLRGswithapeakintheirSEDat25µm.NotallBLRGsshowthisbehaviour, whichdeviatesfromthetypicalSEDforNLRGsandQSRs.Thepeaker sample is compared with a sample of NLRGs and BLRGs which have a peak at 60– 100µm. I find no excess of hot dust in the 25µm peakers, but a lack of cold dust, which is the dust heated by (hot) stars. This might be related to the evolutionary state of the object, being an older NLRG in which the extended dust hasbeen depleted by star-formationandthenuclear dust torusis smaller. The fact that two classes of BLRG seem to exist is not contradicting the unification schemes, however, it adds an important new dimension. Finally, the name 25µm peaker should be abandoned as soon as possible, as it is misleading, but so far I have not found a good alternative. References Barthel P.D., 1989, ApJ 336, 606 Calzetti D., Armus L., Bohlin R.C., et al., 2000, ApJ 533, 682 de Koff S., Baum S.A., Sparks W.B., et al., 1996, ApJS 107, 621 Dennett-Thorpe J., Barthel P.D., van Bemmel I.M., 2000, A&A in press Granato L., Danese G.L., 1994, MNRAS 268, 235 Heckman T.M., Chambers K.C., Postman M., 1992, ApJ 391 39 Hill G.J., Goodrich R.W., DePoy D.L., 1996, ApJ 462, 163 Kessler M.F., Steinz J.A., Anderegg M.E., et al., 1996, A&A 315, L27 Lemke D., Klaas U., Abolins J., et al., 1996, A&A 315, L64 Pier E.A., Krolik J.H., 1992, ApJ 401, 99 Seyfert C.K., 1943, ApJ 97, 28 van Bemmel I.M., Barthel P.D., de Graauw T., 2000, ApJ 359, 523 vanBemmelI.M.,BarthelP.D.,Dennett-ThorpeD.,2001,ApJ,inpreparation 6